California will require the addition of energy storage devices to its electrical grid in an effort to make solar, wind and other renewable-power plants more useful, under a decision Thursday by state utility regulators.

The California Utilities Commission approved requirements for the state's three large investor-owned electric utilities, including San Diego Gas & Electric, to procure 1,325 megawatts of storage capacity by 2020, within installations completed no later than 2024.

The collective output capacity of those storage devices -- spread across transmission, distribution and customer-side wiring -- would be greater than a large nuclear reactor.

Utilities Commissioner Carla Peterman, the main architect of the decision, called Thursday's unanimous vote "an important first step in encouraging the storage market and supporting grid reliability."

The procurement goals are designed to accelerate investment and development in a host of storage technologies, from flow batteries with nearly unlimited longevity to compressed-air storage that pressurizes underground caverns to later spin electrical turbines.

Environmental groups applauded the decision as a step toward meeting peak afternoon and summer power demands without necessarily building new natural gas plants.

Sun and wind power vary dramatically, making it harder to stabilize the grid and match customer demand. Capturing renewable energy for later use is seen as the missing link in the state’s push to generate one-third of its bulk power from renewable sources by 2020 — and ultimately to take a bite out of greenhouse gas emissions and climate change.

Large "pumped storage" hydroelectric facilities, which move water uphill to generate electricity later, will not count toward the new requirements. Peterman said the exclusion was necessary to encourage the development of new, unproven storage technologies.

Beyond the big-name utilites, other electric service providers must acquire storage capable of offsetting 1 percent of their annual peak power demands.

The utilities commission will review competitive solicitations by utilities, including a cost-effectiveness analysis, before energy storage projects can be funded.